1. Field of the Invention
The present invention relates to a blood information measuring apparatus and method for measuring blood information from an image signal of a blood vessel.
2. Description Related to the Prior Art
Endoscopes are widely used for observation of a lesion located inside a human body. The endoscope is provided with an insert section to be introduced into the human body, and a handling section for steering the insert section. The insert section has a lighting window and an imaging window at its distal end. An internal body portion is imaged through the imaging window, while being illuminated with light through the lighting window. An obtained endoscopic image is displayed on a monitor.
As a light source of the endoscope, a white light source such as a xenon lamp or a metal halide lamp is conventionally available. Additionally, there is a method recently in the limelight in which light (narrow band light) of a narrow wavelength band is used as illumination light to facilitate finding out the lesion (refer to US Patent Application Publication No. 2008/0281154 corresponding to Japanese Patent No. 3583731).
Also, there is studied a method for measuring information of blood flowing through a blood vessel, for example, an oxygen saturation level of hemoglobin, a blood flow rate, and the like (refer to Japanese Patent Laid-Open Publication No. 06-315477). In this method, the blood vessel is extracted from the endoscopic image captured under the narrow band light, and the blood information is obtained from an image signal. This method uses the illumination light in wavelength bands of 300 to 400 nm, around 400 nm, 400 to 500 nm, 500 to 600 nm, 450 to 850 nm, and the like. Taking the case of measuring the oxygen saturation level of hemoglobin as an example, an optimal wavelength band is chosen from the five wavelength bands in accordance with the body portion. Each wavelength band has a pair of wavelengths as a wavelength set. The pair includes a measurement wavelength at which absorbance much varies with the oxygen saturation level and a reference wavelength at which the absorbance hardly varies therewith. Two types of light having the measurement and reference wavelengths are applied to the body portion in succession, to obtain a measurement image signal taken under the measurement light and a reference image signal taken under the reference light. The measurement image signal varies based on difference in the absorbance, so the measurement image signal is corrected with the reference image signal to obtain the oxygen saturation level of blood flowing through the blood vessel.
By the way, how deep light penetrates into human tissue depends on a wavelength band of the light. Taking advantage of this property, the depth of a lesion such as cancer can be inspected. More specifically, switching among the wavelength sets makes it possible to measure the oxygen saturation level of blood flowing through blood vessels in different depths from a mucosal layer to a submucosal layer. This allows inspection of the stage of the cancer.
The Japanese Patent Laid-Open Publication No. 06-315477 does not specifically disclose switching timing of the wavelength sets. Furthermore, some types of cancer such as scirrhous carcinoma (linitis plastica) do not manifest themselves in a surface of the mucosal layer, but occur in the middle of the mucosal layer (see FIG. 15). Therefore, the wavelength set suitable for diagnosis differs according to which body portion is to be examined. If the switching among all the wavelength sets is automatically performed, the wavelength sets unnecessary for the diagnosis are inevitably used, resulting in wasted time and effort. If a doctor manually performs the switching, complicated operation is required.